This invention relates to a process for the preparation of a compound having the general formula: ##STR1## wherein R.sup.1 and R.sup.2 each represent an alkyl, an alkenyl or a phenyl group or a hydrogen atom, by non-oxidative dehydrogenation of a compound having the general formula: ##STR2## wherein R.sup.1 and R.sup.2 have the same meaning as in formula I, in which process a mixture comprising a compound of formula II and super-heated steam is contacted at elevated temperature with a catalyst having a spinel structure. The invention also relates to novel compositions and to a process for the preparation of these compositions.
A non-oxidative dehydrogenation is a dehydrogenation whereby no molecular oxygen is added.
An important compound of formula I, styrene, is commercially prepared by dehydrogenation of ethylbenzene in the presence of a catalyst based largely on iron oxide.
A higher conversion of the compounds of formula II and a higher selectivity to the compounds of formula I--compared with the conventional iron oxide catalyst--are achieved in the presence of a catalyst having a spinel structure and containing an alkali metal oxide as promoter, see Canadian Pat. No. 1,072,987. As examples of alkali metal oxides those of sodium, potassium and cesium are mentioned.
A still higher selectivity to the compounds of formula I at the same conversion of the compounds of formula II is achieved in the presence of a spinel catalyst containing an alkali metal oxide and vanadium oxide as promoters, see U.S. Pat. No. 4,220,560, issued Sept. 2, 1980. As examples of alkali metal oxides those of sodium, potassium and cesium are mentioned.
U.S. Pat. No. 3,998,757, issued Dec. 21, 1976, and U.S. Pat. No. 4,067,922 issued Jan. 10, 1978, disclose the use of lithium spinels for use in oxidative dehydrogenation processes. U.S. Pat. No. 3,798,178, issued Mar. 19, 1974, discloses the addition of alkali metal compounds, including lithium to an unsupported iron oxide combined with a finely divided metal such as iron which provides electrical conductivity. There is no mention in this reference of spinel formation. In fact, if the catalyst of this reference were heated to a sufficently high temperature to cause spinel formation, the metal component of the catalyst would be oxidized, destroying the desired high electrical conductivity.
The selectivity to a certain compound, expressed in a percentage, is defined as EQU (a/b).times.100
wherein "a" is the amount of the compound of formula II that has been converted into that certain compound and "b" is the total amount of the compound of formula II that has been converted.
The above known processes may be carried out in the presence of from 2 to 20 mol of steam per mol of starting compound of formula II, typically for example of 12 mol/mol.
A process has now been found which gives even higher selectivities to the compounds of formula I (at the same conversion) than the process described in U.S. Pat. No. 4,220,560, but at considerably a lower ratio of steam to the starting compound of formula II and at a lower temperature. Hence, a corresponding reduction in steam costs has been achieved.